One of the basic tenets of invasion biology is “ecological niche theory.” According to that theory, every species occupies a specific niche to which it is adapted. That niche provides all the biological resources the species needs for its survival and reproduction.
A corollary to that theory is that when non-native species are introduced to that “niche” the native occupant is the loser in the inevitable competition for available resources. The intruder has the advantage in this competition because its predators have not usually been introduced at the same time. This is the “predator release” corollary.
Adherents to ecological niche theory therefore routinely predict the demise of native species whenever non-native species are introduced. Often, their belief in this inevitable competition leads them to see what they expect to see. The prediction that the introduction of the European honeybee to the New World would eventually decimate populations of native bees is an example of this mindset. Since honeybees were introduced to the New World over 400 years ago, it seems reasonable to expect to see some evidence of this consequence by now.
Seeing what we expect to see
Forgotten Pollinators was published in 1996, at the height of popularity of invasion biology. (1) We consider it a valuable, interesting book, but we were not impressed with the chapter devoted to the belief of the authors that the European honeybee is competing with native pollinators, to the detriment of native pollinators. Although the authors interviewed several other scientists who shared that belief, they were unable to offer any empirical evidence that supported their belief.
One of the studies cited in Forgotten Pollinators quantified the amount of nectar and pollen consumed by honeybees and compared that to the quantity of nectar and pollen required by native bumblebees. Based on those calculations, they predicted the demise of bumblebees based entirely on the amount of nectar and pollen consumed by honeybees. The study arrived at the preposterous conclusion that a single honeybee hive could reduce the population of bumblebees by 38,400.
This dire prediction is based on the assumption that there is a finite amount of pollen and nectar available. Therefore, every scrap of food collected by a honeybee is a scrap of food taken from a bumblebee. It also assumes that the bumblebee loses the competition 100% of the time and neither insect is capable of expanding its range in the unlikely event that there is in fact a finite amount of food available. This type of “zero-sum” thinking pervades the nativist ideology, e.g., a job taken by an immigrant is presumed to be taken from a resident.
Looking for bad news…finding good news
The federal government has invested in many careers and large sums of money to prove the assumptions of invasion biology as well as funding eradication projects based on those assumptions. For example, the University of Alaska, Fairbanks was awarded a federal grant for $493,000 to test the hypothesis that the existence of non-native sweetclover is drawing pollinators away from wild native food sources such as cranberry and blueberry bushes. After several years, researchers have concluded, “…there’s usually room for everybody.” (2)
The researchers monitored 20 sites for two years. They observed that the sweetclover patches “were actually attracting three times as many pollinators to native plants as they would otherwise get.” The sweetclover attracted many different types of pollinators, including moths, flies, and wasps. Consequently there were many more cranberries in the “invaded” patches and equal quantities of blueberries.
Suspicious of those findings, the researchers moved their project to a controlled setting. Then they got mixed results, which seemed to depend upon variations in the weather.
Researchers are still intent to find negative consequences of the existence of sweetclover. Now they are hoping to prove that the sweetclover is changing the composition of the soil, which they predict will eventually “crowd out” the native plant species. Will they keep looking until they can report bad news?
Empirical evidence is absent
After reading this good news about the exoneration of sweetclover in the nativist blame game, we decided to revisit the accusations made by Forgotten Pollinators about honeybees causing the decline of native bee populations. Our search of scientific literature published after the publication of Forgotten Pollinators in 1996, was very revealing and is best represented by a review article published in 2004, “Impact of the introduced honeybee on native bees: A review.” (3)
The review analyzes 28 studies conducted all over the world about the impact of honeybees on populations of native bees. This is a summary of the analysis:
“Although previous studies investigating indirect measurements have been cited as evidence of competition between honey bees and native bees, many of these studies were compromised by low replication, confounding factors or poor interpretation. Studies that are well designed and implemented may find the potential to impact negatively on native bees but the use of indirect measurements does not reveal [their impact on] long-term survival of native bees.
More direct studies of the impact of honey bees on native bee survival, fecundity or population density have shown little evidence that the presence of honey bees has any impact on native bees.” (3)
As we often do on Million Trees, we conclude with this rhetorical question: How does invasion biology survive in the absence of empirical evidence that supports its hypothetical assumptions?
(1) Stephen Buchmann & Gary Paul Nabhan, Forgotten Pollinators, Island Press, 1996
(2) “Invasives pollination study shows mixed results for Alaska berries,” Fairbanks Daily News-Miner, August 23, 2013
(3) D.R. Paini, “Impact of introduced honey bee (Apis mellifera) on native bees: A review,” Austral Biology, (2004) 29, 399-407.